Dynamic space check for multi-arm system moving on a rail

ABSTRACT

A system and method for controlling motion interference avoidance for a plurality of robots are disclosed, the system and method including a dynamic space check system wherein an efficiency of operation is maximized and a potential for interference or collision is minimized.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/791,724 filed on Apr. 13, 2006, hereby incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to a system for controlling a plurality of robots and a method for controlling motion interference avoidance for the plurality of robots.

BACKGROUND OF THE INVENTION

Movement of objects in space is a necessary task in a typical manufacturing environment. Robotics have increasingly been employed to effect the necessary movement. However, when multiple objects are being moved, a potential for interference between the objects exists. An interference exists if the at least two objects share the same space at the same time. That is, the objects have the same coordinates with respect to a common frame of reference.

With modern industrial robots moving at considerable velocities, interferences between robots can result in a collision and undesirable damage to the robots and work pieces being handled by the robots. The collisions may lead to costly down time in the manufacturing process. Accordingly, it is desirable to avoid such collisions.

Prior art systems and methods have been used in an attempt to minimize interferences and collisions. However, there are several shortcomings of the prior art systems and methods. Typically, a tool center point (TCP) is only checked relative to a predetermined interference area (static space). For multiple robots, it is difficult to directly or effectively prevent the collision or interference thereof. Further, it is difficult to specify an interference space in respect of a static coordinate system for multiple moving robots. Any interference space is not only a function of the robot motion path, but also a function of the motion speed. Difficulty also exists in attempting to handle a deadlock situation when two or more robots request to move to a common space at the same time.

Prior art systems also attempt to prevent a TCP for a robot from colliding in a fixed space relative to its world coordinate system. When multi robots (with multiple controllers) share common spaces or so called interference spaces during a task execution, each controller has to wait until no robot is in the common spaces, and then the controller can issue the motion control commands to allow the robot to move. This process is also called a wait and move process, which generally increases working cycle time. Draw backs exist when using the prior art systems with multiple robots. It is difficult to effectively specify an interference space in terms of a fixed coordinate system, because the interference space is not only the function of the robot motion path but also the motion speed. When more than one robot requests to move to a common space at the same time, it creates a deadlock situation where none of the robots can move because they are waiting for one another.

It would be desirable to have a system and method for controlling motion interference avoidance for a plurality of robots, wherein an efficiency of operation is maximized and a potential for interference or collision is minimized.

SUMMARY OF THE INVENTION

Consistent and consonant with the present invention, a system and method for controlling motion interference avoidance for a plurality of robots, wherein an efficiency of operation is maximized and a potential for interference or collision is minimized, has surprisingly been discovered.

In general terms, the invention relates to a system and method of controlling motion of a plurality of robots on the rail and a plurality of the controllers, wherein robots controlled by each controller will only work on a specified dynamic space, thus avoiding a collision. Control objectives are realized by defining a set of dynamic spaces for each controller, an ability to monitor the dynamic spaces in real time, and control mechanisms that are able to hold or unhold a motion of the robot controlled by each individual controller to militate against interference between robots within the dynamic space.

In one embodiment, the system for controlling motion interference avoidance for a plurality of robots comprises a plurality of controllers adapted for connection to a source of power; a sequence of instructions residing on the controller for execution thereon, the sequence of instructions including a dynamic space check control method; a plurality of robots in communication with and controlled by the controllers; and a programmable logic device in communication with the controllers.

The invention also provides methods for controlling motion interference avoidance for a plurality of robots.

In one method according to the invention comprises the steps of providing a plurality of controllers adapted for connection to a source of power, a plurality of robots in communication with and controlled by the controllers, and a programmable logic device in communication with the controllers; and providing a sequence of instructions including a dynamic space check control method residing on the controller for execution thereon, the dynamic space check control method further comprising the steps of setting up dynamic spaces for each of the robots; updating the dynamic spaces; detecting the dynamic spaces; measuring interferences; a motion hold command; a motion release command; and a deadlock decision.

In another method according to the invention comprises the steps of providing a plurality of controllers adapted for connection to a source of power, a plurality of robots in communication with and controlled by the controllers, and a programmable logic device in communication with the controllers; and providing a sequence of instructions including a dynamic space check control method residing on the controller for execution thereon, the dynamic space check control method further comprising the steps of setting up dynamic spaces for each of the robots including one of a spherical method, a cylindrical method, and a box method; updating the dynamic spaces including sending updated reference information from one of the controllers to other of the controllers via a network media at an interpolated interval; detecting the dynamic spaces including receiving the updated reference information from neighboring controllers at the interpolated interval; measuring interferences including reconstruction of the dynamic space of the neighboring controllers and calculation of a minimal distance between the dynamic space of the neighboring controller and the dynamic space of the one of the controllers, wherein the step of calculation of a minimal distance between the dynamic space of the neighboring controller and the dynamic space of the one of the controllers incorporates time, distance, and one of mechanical inertia of acceleration and deceleration; a motion hold command; a motion release command; and a deadlock decision.

DESCRIPTION OF THE DRAWINGS

The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which:

FIG. 1 is a schematic diagram showing an example of a plurality of robots on a rail with a network media and PLC connected to each controller;

FIG. 2 shows three examples of modeling method for dynamic spaces that are relative to the TCP position;

FIG. 3 shows a typical example of setting up the dynamic spaces for a series of the top loader robots on the rail;

FIG. 4 illustrates the concept of the TRACE of the dynamic space; it also shows how to calculate the minimal distance between the two dynamic spaces in terms of the trice of the dynamic space;

FIG. 5 illustrates an example of automatic motion hold and unholds control flow with two controllers;

FIG. 6 shows the deadlock prevention in different situations; and

FIG. 7 is a schematic flow diagram showing a dynamic space check method according to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals also indicate like or corresponding parts and features. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, are not necessary or critical.

FIG. 1 is a schematic diagram showing a robotic system 10 known as a rail application. The robotic system 10 includes a plurality of robot controllers 12 for controlling at least one of a plurality of robots 14, the controllers in electrical communication with a source of electrical power. In the embodiment shown, each of the controllers 12 includes an associated motion system (not shown) to control two attached top loader robots 14 designated as R1 and R2, although other types and configurations can be used as desired. The controllers 12 are in communication with one another by a computer communication network 16 with a programmable logic device (PLC) 18 and are connected by network media 20. Each of the controllers 12 can execute a sequence of an instruction or program residing within the controller 12. In the rail application, the robots 14 move along a rail 22 and can perform independent or in a coordinated motion.

FIG. 7 shows a dynamic space check control method 24 according to an embodiment of the invention. In general, the control method 24 involves several steps. One step of the dynamic space check control method 24 involves setting up a dynamic space 26 for each controller 12. There are a variety of ways to set up a dynamic space in terms of a geometric model and a reference point. In FIG. 2, three methods for setting up a dynamic space 26 are shown: a spherical method 28; a cylindrical method 30; and a box method 32. The methods 28, 30, 32 shown are for dynamic spaces that are relative to a tool center point (TCP) position, although other methods can be used as desired. Additionally, other shapes and configurations can be used without departing from the scope and spirit of the invention. Each modeling utilizes a set of parameters. For the spherical method 28, TCP, an offset OFFSET from the TCP, and a radius R of the sphere are used. For the cylindrical method 30, TCP, an offset OFFSET from the TCP, a height of the cylinder H, and a radius of the cylinder R are used. TCP, an offset OFFSET from the TCP, a height of the box H, a length of the box L, and a width of the box are used for the box method 32. For purposes of the method disclosed herein, it is assumed that all of the methods 28, 30, 32 have the same orientation as the TCP position. It is understood that other configurations can also be used.

FIG. 3 illustrates a method of setting up the dynamic spaces 26 for the robots 14. For purposes of illustration, it is assumed that all of the robots 14 have a common world coordinate frame and the world coordinate Y-axis is aligned with a longitudinal axis of the rail 22. Each controller 12 is controlling the pair of robots 14 designated by R1 and R2. A first axis of each robot 14 is aligned with the rail 22. A set of dynamic spaces for each controller 12 is defined by a low boundary LB and a high boundary HB relative to a Y-axis position of the robot 14 designated by R1 represented by an offset 1 OFS1 and an offset 2 OFS2, respectively. The low boundary LB and the high boundary HB are defined as planes that are substantially perpendicular to the rail 22 and move along the first axis of the robot 14 designated by R1. Therefore, the low boundary LB and the high boundary HB specify a space region for each controller 12. A minimal distance between the high boundary HB (or the low boundary LB) to the low boundary LB (or the high boundary HB) of a neighboring controller is designated DIST. A motion of the robots 14 is controlled by the different controllers 12 so that DIST is always greater than a margin distance, thus militating against an interference among the dynamic spaces.

Another step of the dynamic space check control method 24 involves updating the dynamic spaces 34 during a task execution. Because the set up dynamic space 26 is attached to a reference frame or a position, the set up dynamic space 26 dynamically changes during movement of the robot 14. Each controller 12 sends updated reference information to the other controllers 12 via the network media 20. The updated reference information is sent in a selectable interval of time, normally at an interpolated interval ITP. For the top loader robot 14 application, the reference information is typically a Y coordinate position (a position on the rail) of one of the robots 14 designated by R1 for each of the controllers 12. The reference information is sent to a neighboring controller 12 via the network media at every interpolated interval ITP.

Next, the dynamic spaces are detected 36 across the controllers 12. Each controller 23 receives the updated reference information from other controllers at a desired interval. Typically, the desired interval is a multiple interval of the interpolated interval ITP, depending on a capacity of the network media 20. For the top loader robot 14 application, each controller receives the updated reference information of the neighboring controllers 12 at every interpolated interval ITP. Based on the updated reference information, each controller 12 calculates the dynamic spaces, the low boundary LB, and the high boundary HB for the neighboring controllers 12 with respect to its own reference frame.

A measurement of interferences 38 of the dynamic spaces is then determined between the controllers 12. The measurement of interferences 38 step further involves reconstruction of the dynamic space of the neighboring controller 12 and a calculation of the minimal distance DIST between the dynamic spaces. The distance measured between the dynamic spaces incorporates mechanical inertia of acceleration or deceleration, time, and distance in order to effectively militate against interference between the dynamic spaces. In order to illustrate the measurement, the following terminologies are used:

-   -   Current position (cur_pos) refers to an instantaneous         measurement of a position at the present time.     -   Current frame (cur_frm) refers to an instantaneous measurement         of a frame at the present time.     -   Command position (cmd_pos) refers to an interpolated position at         the present time that the robot intents to move to.     -   Command frame (cmd_frm) refers to a coordinate frame at the         present time, derived from a set of cmd_pos.     -   Current dynamic space (cur_ds) refers to a space occupation at         present time derived from one or set of cur_pos or cur_frm.     -   Command dynamic space (cmd_ds) refers to a space occupation at         present time derived from one or set of cmd_pos or cmd_frm.     -   Trace of the dynamic space (tr_ds) refers to a total dynamic         space occupation from cur_ds to the cmd_ds.     -   Minimal distance of two dynamic spaces is defined by minimal         distance of their traces of the dynamic spaces.

FIG. 4 illustrates how the distance DIST of the dynamic space is calculated in terms of a TRACE of dynamic space for different configurations, where C1 and C2 represent respective first and second controllers 12, and each cube represents a TRACE of a dynamic space. Left and right cubes for each configuration represent respectively the dynamic space HB for C1 and LB for C2.

-   -   Configuration 1, R1 for C1 and C2 moving in +Y (move same         direction), DIST equals the distance from cmd_ds of C1 to cur_ds         of C2.     -   Configuration 2, R1 for C1 and C2 move toward each other. DIST         equals the distance from cmd_ds of C1 to cmd_ds of C2.     -   Configuration 3, R1 for C1 and C2 moves away from each other.         DIST equals the distance from cur_ds of C1 to cur_ds of C2.

A motion hold command 40 is issued by the controllers 12 to stop the robots 14 at command positions whenever the measured minimal distance DIST between the dynamic spaces equals or is less than a specified marginal distance. The hold command militates against the minimal distance DIST being less than the marginal distance.

A motion unhold command or motion release command 42 is issued by the controllers 12 once the measured minimal distance DIST is greater than the specified marginal distance. Accordingly, all robots 14 being held resume motion. FIG. 5 illustrates an example of how the motion hold command and the motion unhold command control flow between two controllers 12. Each controller 12 independently conducts the dynamic space calculation 36 and the distance measurement 38, and makes a motion control decision. However, each controller 12 sends or receives reference information to or from the other controller 12 such that dynamic space is calculated based on updated reference information. Data sending or receiving is accomplished via the network 16 connected between the controllers 12.

The dynamic space check control method 24 includes a step of making a deadlock decision 44. Typically, when a potential interference is detected, the motion hold command 40 is issued to the robots 14. To prevent a deadlock, where the robots will not resume motion, the deadlock decision step 44 is activated. The deadlock decision step 44 checks the measured distance DIST of the dynamic space and checks a rate of change of the measured distance DIST. If the motion of the robot 14 will result in an increase of the measured distance DIST from the previous measured distance DIST, then the robot 14 will be issued the motion release command 42, even if the current measured distance DIST is less than the marginal distance. FIG. 6 depicts the control logic.

-   -   The trailing robot in case A and B of FIG. 6 will be held until         the interference is cleared due to the motion of the leading         robot.     -   The leading robot in case A and B of FIG. 6 will not be held         because the motion of the leading robot will clear the dynamic         space interference for the trailing robot.     -   If neighboring robots move toward each other as in C of FIG. 6,         the system will hold both robots (or both pairs) upon the         detection of the space interference. Manual intervention is         required to clear this situation.     -   If one robot is stationary while the other robot (or robot pair)         is moving toward the stationary robot as in D of FIG. 6, the         system will hold the moving robot (or robot pair) only.

The deadlock decision step 44 allows an operator to get out the deadlock situation as quickly as possible. In the meantime, the dynamic space check control method 24 to militate against the deadlock situation also maximizes a production cycle efficiency.

The dynamic space check control method 24 can be easily defined in terms of a specified moving frame or reference point of interest relative to the robot 14 within the controller 12. The dynamic space check control method 24 is dynamically updated with its reference frame or reference point during task execution. Additionally, the dynamic space check control method 24 can be easily represented across controllers 12 because it is easier to pass a reference frame or a reference point from one controller 12 to another controller 12 via network media 20 than to pass a geometric space. Interference avoidance is also facilitated for multiple moving robots 14. The motion hold command and the motion release command of the dynamic space check control method 24 also optimizes a cycle time for the robots 14. The dynamic space check control method 24 also voids using hardware to achieve the required functionality.

For coordinated motion between groups of robots 14 controlled by a common controller 12, a control system known as a space control manager system is provided within each common controller 12. The space control manager system controls traffic or priority for a common space accessed by one of the robots 14 or group of robots 14. The traffic or priority can be controlled for each common spaced accessed, within or across controllers 12, group motion, or independent motion. This facilitates additional functionality for multiple robots to access a common space. The additional functionality includes support for multiple robot 14 groups dynamic space check control within one controller 12, multiple robot 14 groups across the controllers 12, multiple robot 14 groups teach pendent (TP) program execution, concurrent single robot 14 group TP program execution, multiple robot 14 group TP program and single robot 14 group TP program execution, coordinated motion, and jog operation for multiple group motion.

In order to facilitate the additional functionality, additional control methods are provided within the controllers 14. For multiple robot 14 groups dynamic space check control within one controller 12, the common space control manager controls the priority for involved groups dynamically for each common space Priority of the group motion is determined according to the following rules. Priority is assigned on a first come, first served basis. Thus, the group arriving the earliest according to a waiting queue is assigned the highest priority. For group motion or coordinated motion, all the robots 14 within the groups have the same priorities when the groups are in the waiting queue waiting for a space to be cleared.

For multiple robot 14 groups across the controllers 12 accessing a common space, the controller 12 whose group or groups of robots 14 occupy the space has a higher priority than another controller 12. The space control manager determines a priority of its own controller 12 based on the status of the other controller 12, as well as the status of its own controller 12. When its own controller has a higher priority, dynamic space check control within one controller 12 is followed. When no group occupies the common space and no group is in the waiting queue, the space control manager's own controller surrenders control, which allows the other controller 12 to take control if a group or groups are waiting in the queue to access the common space. If two robots 14 from different controllers 12 intend to access the common space at the same time, the controller with a higher priority assignment will be permitted to access the common space first.

The space control manager controls a state transition for each of multiple robot 14 groups that access a common space. For each state transition, the common space manager takes an action within motion sub-systems interrupt (INTR) or filter (FLTR) tasks to verify that group motion requirements will not be violated. For movement OUT→IN, access to the space is permitted, and the space is designated as occupied. In respect of movement OUT→WAIT, access to the space is not permitted, motion is held, and the waiting priority for the motion group is updated in the queue. For WAIT→IN, access is permitted into the space, the group motion is released, and the waiting priority for the motion group in the queue is updated. An occupation designation for the space is cancelled for IN→OUT movement. For movement IN→IN and OUT→OUT, motion is not restricted since the space status is not changed.

For concurrent single robot 14 group TP program execution, the space control manager assigns the same priority to each group member in the group to permit access to the common space. When one of the groups has to wait due to an occupied space, all of the groups within the group are held simultaneously. The space control manager allows the holding group or groups to complete an interpolated position for a current interpolation period (ITP) to maintain the group motion relationship while all groups stop at a held position.

When releasing groups when the space becomes available, the space control manager releases the motion simultaneously. This allows the group motion relationship to be maintained while motion release is occurring.

When using an interpolated position to check the space, the TCP is caused to wait inside of the space because the space control manager allows the interpolated point to complete. Causing the TCP to wait inside of the space may produce an ambiguous state, and result in a race condition after power is restored. It is more desirable to cause the TCP to wait outside of the space. At each interpolated stage, the TCP is determined by the actual command position of the TCP and the delta position calculated by the speed and a predication rate. Accordingly, the TCP can be stopped prior to actually entering the space using the TCP calculated according to this method.

The primary deadlock control involves prevention as well as easy handling if it does occur. Ideally, the motion control manager will not allow the deadlock to occur in normal operation condition. However exception may occur under following conditions: the space is enabled dynamically while multiple robots are in the same space; a program is aborted due to an error while multiple robots are in the same space; and a TP program is started while multiple robots are in the same space. The space control manager facilitates a recovery if a deadlock occurs. Any deadlock groups can be jogged if inside of a common space. Once the the group is jogged outside of the deadlocked space, the group cannot be jogged inside the space again unless the space is cleared. Non-deadlocked groups outside of the space can be jogged, but the non-deadlocked groups cannot be jogged into a deadlocked space until the space is cleared. A TP program involving groups not deadlocked can be executed. However, these groups involved in the TP program cannot enter the deadlocked space during execution of the TP program. Additionally, if an attempt is made to execute a TP program with groups involved in the deadlock, the system will abort the program and return to deadlock. Finally, if the deadlock involves groups from two controllers, recovery of the deadlock from either controller will recover the other controller from deadlock as well.

In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope. 

1. A system for controlling motion interference avoidance for a plurality of robots comprising: a plurality of controllers adapted for connection to a source of power; a sequence of instructions residing on said controller for execution thereon, said sequence of instructions including a dynamic space check control method; a plurality of robots in communication with and controlled by said controllers; and a programmable logic device in communication with the controllers.
 2. The system according to claim 1, wherein said sequence of instructions includes setting up dynamic spaces for each of said robots, updating the dynamic spaces, detecting the dynamic spaces, measuring interferences, a motion hold command, a motion release command, and a deadlock decision.
 3. The system according to claim 2, wherein the setting up dynamic spaces includes one of a spherical method, a cylindrical method, and a box method.
 4. The system according to claim 2, wherein the setting up dynamic spaces includes a low boundary and a high boundary relative to a position of each of said robots.
 5. A method for controlling motion interference avoidance for a plurality of robots, the method comprising the steps of: providing a plurality of controllers adapted for connection to a source of power, a plurality of robots in communication with and controlled by the controllers, and a programmable logic device in communication with the controllers; and providing a sequence of instructions including a dynamic space check control method residing on the controller for execution thereon, the dynamic space check control method further comprising the steps of: setting up dynamic spaces for each of the robots; updating the dynamic spaces; detecting the dynamic spaces; measuring interferences; a motion hold command; a motion release command; and a deadlock decision.
 6. The method according to claim 5, wherein said step of setting up dynamic spaces includes one of a spherical method, a cylindrical method, and a box method.
 7. The method according to claim 6, wherein said step of setting up dynamic spaces including the spherical method utilizes a tool center point, an offset from the tool center point to a center point of a sphere defining one of the dynamic spaces, and a radius of the sphere defining the one of the dynamic spaces.
 8. The method according to claim 6, wherein said step of setting up dynamic spaces including the cylindrical method utilizes a tool center point, an offset from the tool center point to a center point of a base of a cylinder defining one of the dynamic spaces, a height of the cylinder defining the one of the dynamic spaces, and a radius of the cylinder defining the one of the dynamic spaces.
 9. The method according to claim 6, wherein said step of setting up dynamic spaces including the box method utilizes a tool center point, an offset from the tool center point to a corner point of a base of a box defining one of the dynamic spaces, a height of the box defining the one of the dynamic spaces, a length of the box defining the one of the dynamic spaces, a width of the box defining the one of the dynamic spaces.
 10. The method according to claim 5, wherein said step of setting up dynamic spaces includes the steps of defining a low boundary and defining a high boundary relative to a position of each of the robots on an axis.
 11. The method according to claim 5, wherein said step of updating the dynamic spaces includes the step of sending updated reference information from one of the controllers to other of the controllers via a network media at an interpolated interval.
 12. The method according to claim 11, wherein said step of detecting the dynamic spaces includes the step of receiving the updated reference information from neighboring controllers at a desired interval.
 13. The method according to claim 12, wherein said step of measuring interferences includes the steps of reconstruction of the dynamic space of the neighboring controllers and calculation of a minimal distance between the dynamic space of the neighboring controller and the dynamic space of the one of the controllers.
 14. The method according to claim 13, wherein the step of calculation of a minimal distance between the dynamic space of the neighboring controller and the dynamic space of the one of the controllers incorporates time, distance, and one of mechanical inertia of acceleration and deceleration.
 15. The method according to claim 13, wherein the step of calculation of a minimal distance between the dynamic space of the neighboring controller and the dynamic space of the one of the controllers is calculated in terms of a trace of dynamic spaces.
 16. The method according to claim 5, wherein each of the controllers includes a space control manager system to facilitate coordinated motion between groups of robots controlled by a common one of the controllers.
 17. The method according to claim 16, wherein the space control manager system controls priority for one of a common space accessed by a group of the robots, across at least two of the controllers, group motion, and independent motion.
 18. The method according to claim 16, wherein the space control manager system assigns priority for occupying a space based on at least one of a first come, first served basis and which robot has a higher priority.
 19. A method for controlling motion interference avoidance for a plurality of top loader robots on a rail system, the method comprising the steps of: providing a plurality of controllers adapted for connection to a source of power, a plurality of robots in communication with and controlled by the controllers, and a programmable logic device in communication with the controllers; and providing a sequence of instructions including a dynamic space check control method residing on the controller for execution thereon, the dynamic space check control method further comprising the steps of: setting up dynamic spaces for each of the robots including one of a spherical method, a cylindrical method, and a box method; updating the dynamic spaces including sending updated reference information from one of the controllers to other of the controllers via a network media at an interpolated interval; detecting the dynamic spaces including receiving the updated reference information from neighboring controllers at the interpolated interval; measuring interferences including reconstruction of the dynamic space of the neighboring controllers and calculation of a minimal distance between the dynamic space of the neighboring controller and the dynamic space of the one of the controllers, wherein the step of calculation of a minimal distance between the dynamic space of the neighboring controller and the dynamic space of the one of the controllers incorporates time, distance, and one of mechanical inertia of acceleration and deceleration; a motion hold command; a motion release command; and a deadlock decision.
 20. The method according to claim 5, wherein each of the controllers includes a space control manager system to facilitate coordinated motion between groups of robots controlled by a common one of the controllers. 